Use and Limitation Edit
Rei has a unique ability far more than any other. Rei has the ability to turn those of human to altered or simply disintegrate other altered by physical touch. As Rei himself states, when 'Altering' he is able to choose which ability to write into human or altered which allows him to give and change abilities the full extent of this is unknown. It's possible for Rei to alter his own abilities which he apparently used to give himself wings to fly with and was shown using it in the final chapter.
The only so far known limitation is that Rei has to physically touch in order to alter or disintegrate person.